Luffy's Fall! The Unexplored Region - Grand Adventure in the Ocean's Navel (2000)

The Straw Hat Pirates stumble upon an enormous hole in the ocean, at the bottom of which lies a mysterious island known as the "Ocean's Navel". The Straw Hat Pirates resolve to find the island's treasure, which can supposedly grant any wish, but the island's inhabitants possess agendas of their own for their new guests...